This invention relates to arc welding and, more particularly, the present invention is directed to an apparatus which utilizes a preheating step whereby the wire heat content is increased by means other than welding current.
In conventional gas-metal arc welding (GMAW) processes, heating of the alloyed wire prior to deposition is accomplished by passing welding current through a certain wire length, commonly referred to as the wire stick-out. The power consumed in heating this wire is equal to the product of the square of the welding current and the resistance of the wire. In order to increase the wire deposition rate, the heat content of the wire is increased by increasing the electrical stick-out or the welding current or both.
Excessive wire stick-out leads to uncontrolled wire wandering and/or poor deposit quality. Therefore, welding current is the predominant variable that determines deposition rate and the mode of metal transfer through the arc. However, current intensification leads to more power dissipation in the arc. Since approximately 65 to 85 percent of the arc heat is conducted into the metal base, a higher current would increase arc penetration while increasing deposition rate and decreasing dilution. Although dilution can be reduced by employing higher welding current and slower speeds of travel, these means of control have practical limitations. Higher heat input per unit of length can generate excessive assembly distortion and metallurgical damage in both the deposit and base metal, such as heat affected zone (HAZ) underbead cracking and hot cracking.
In the conventional hot wire gas-tungsten arc welding (HWGTAW) process, heating of the alloyed wire prior to deposition is accomplished by passing heating current through a certain length of wire stick-out. As in the GMAW process, higher wire heat content is adjusted by increasing wire stick-out or increasing heating current or both. This makes the HWGTAW process subject to similar difficulties experienced by the GMAW process. U.S. Pat. No. 4,447,703, which is assigned to the assignee of the present invention and incorporated by reference herein, teaches a technique for overcoming the aforedescribed difficulties. The aforesaid patent describes a technique for preheating the wire at a location remote from the arc and molten pool, thereby reducing the required welding current and wire stick-out length. Thus, in a gas-metal arc welding method in which a power supply feeds welding current through a welding wire electrode into a metal workpiece, the aforesaid patent adds a preheating step in which the wire heat content is increased by means other than the welding current. By increasing the heat content of the wire, a significant reduction in weld dilution is achieved. This preheating is accomplished by passing a current through a wire segment prior to the wire's entry into the stick-out region. The use of preheating the wire limits the length of wire stick-out required to reach a desired wire temperature for a given current in the wire, thereby minimizing wire wandering, improving deposit quality and minimizing arc-wire interaction.
It is therefore an object of this invention to provide an improved concept for electrically isolated hot wire welding, cladding, brazing and/or surfacing processes.
It is also an object of this invention to provide a technique which permits consistent preheating of the wire by passing current between electrical contact tips.
It is a further object of this invention to provide a technique whereby electrical contact consistency between the preheated wire and the electric tips is maximized.